Anchors have a number of applications in the construction industry. One exemplary use is in securing building foundations to the frame of the building. Building foundations transfer structural loads from the building safely to the ground. Foundations are subject to a number of different types of loads, including the dead load of the building, live loads such as of people, furnishings and changing elements in the building, wind loads, horizontal pressures of earth and water, and forces resulting from earthquakes. These loads can affect the building differently. For example, wind loads apply lateral, downward and uplift forces while earthquakes can apply horizontal and vertical forces.
To help secure the foundation to the building, anchors are embedded into concrete foundations to fasten a building frame to a foundation. Anchors can be used in the initial construction of a building or in retrofitting older building. Retrofit anchors, also referred to as post-installed anchors, may consist of an externally threaded portion at one end of the anchor and an insert portion another end of the anchor. The insert portion is designed to be installed in a pre-drilled hole in the concrete or masonry foundation. The insert portion of the anchor is then placed in the pre-drilled hole and set in the hole through the use of a strong epoxy adhesive or a mortar composition which binds to the concrete or masonry and the insert.
Due to the increasing use of post installed anchors, building codes now directly address the design and use of post-installed anchors. There are numerous designs which attempt to improve the performance of pre and post installed anchors have been conceived.
One prior art design is shown in FIG. 1. FIG. 1 illustrates a prior art insert rod such as that manufactured by Hilti Corporation under the name Hit-TZ. The insert rod 100 includes a helical insert section 125 and a threaded section 130. The helical section is designed to be inserted into a pre-drilled hole into which a mortar or other adhesive is provided to secure the insert rod in the hole. The hole is provided in a concrete or masonry block 110 and mortar or adhesive 140 surrounds the helical section 136. The building element 120 is secured to the concrete 110 by a nut 146 and washer 145, the nut having a thread which matches the thread 135 on threaded section 130 of insert 100.
The insert 100 is designed with a relatively smooth surface so that if the insert rod is installed in a section of mortar or concrete 110 which in which a crack 150 subsequently forms, any failure of bond will occur at the interface between the insert rod surface and the adhesive, in addition to any bond failure between the adhesive and the concrete 110.
If the failure occurs at the interface between the rod and the adhesive, it has been noted that the high pitch of the helical section of such inserts gives rise to a tendency of the bolt to unthread from the adhesive section under cyclic loads. This can result in a failure of the insert rod to secure a building element 120 to the concrete 110.